Method of isolating Streptococcus sobrinus and selection medium

ABSTRACT

A method of selectively isolating  Streptococcus sobrinus  alone out of oral streptococci including mutans streptococci, and a medium for selecting  Streptococcus sobrinus  are developed. 
     A method of substantially isolating  Streptococcus sobrinus  alone by the addition of a monobactam antibiotic to a medium on which only oral streptococci including mutans streptococci can grow; and a selective medium for  Streptococcus sobrinus  prepared by the addition of a monobactam antibiotic to a medium on which only oral streptococci including mutans streptococci can grow, are provided with.

This application is a National Stage Application filed under Rule 371 based on PCT/JP02/06822 filed Jul. 4, 2002, which claims priority to Application Japan 2001 227137 filed Jul. 27, 2001.

TECHNICAL FIELD

The present invention relates to a method of selectively isolating Streptococcus sobrinus out of dental caries-causing bacteria (mutans streptococci) inhabiting in the oral cavity, and a medium which allows Streptococcus sobrinus alone to selectively grow.

BACKGROUND ART

Heretofore, it has been said that the bacteria which cause dental caries in human is Streptococcus mutans out of streptococci (oral streptococci) inhabiting in the oral cavity. However, it is considered nowadays that dental caries is caused by a group of bacteria generically called as mutans streptococci which is constituted by bacterial species distinguished from each other in the different serotypes a to h (Hamada, S. and H. D. Slade, 1980, Microbiological Review 44:331–384; Hirasawa, M. et al., 1980, Infect. Immun. 27:1003–1011; and Loesche, W. J., 1986, Microbiological Review 50:353–380). Oral streptococci classified into mutans streptococci include Streptococcus cricetus, Streptococcus rattus, Streptococcus mutans, Streptococcus sobrinus and Streptococcus downeii.

Further, the recognition is recently being accepted that Streptococcus mutans and Streptococcus sobrinus out of the bacteria classified into mutans streptococcl play particularly important roles in dental caries. Particularly, in Europe, the significance of Streptococcus sobrinus attracts attention, and there are reports that caries risk in the case where both of Streptococcus mutans and Streptococcus sobrinus inhabit in the oral cavity becomes higher than that in the case of Streptococcus mutans alone (Babaahmady, K. G. et al., 1998, Caries Res. 32:51–58; Kohler, B. and S. Bjarnason, 1987, Community Dent. Oral Epidemiol. 15:332–335; and Lindquist, S. and C. G. Emilson, 1991, Caries Res. 25:146–152). However, the distribution of Streptococcus mutans and Streptococcus sobrinus in dental caries, and the relative significance thereof to dental caries have not been clarified.

In Japan, it has been said that mainly found out the clinical samples of dental caries (carious cavity, dental plaque and saliva) is Streptococcus mutans. However, it is now discussed by the researchers how many Streptococcus sobrinus inhabit the oral cavity, and, whether or not the presence of Streptococcus sobrinus is really important for caries risk.

Mitis Salivarius Agar (MS, manufactured by Difco) is conventionally used as a selective medium for streptococci in the oral cavity. A medium prepared by the addition of sucrose and an antibiotic bacitracin to the Mitis Salivarius Agar is used as a selective medium for mutans streptococci (MSB medium). However, it is known that in the MSB medium, the recovery of Streptococcus sobrinus is lower than that of Streptococcus mutans. Further, Streptococcus mutans and Streptococcus sobrinus are closely resemble to each other in their colony morphology and so on, therefore, unless a researcher is conversant with the colony morphology of these species, under the circumstances, he or she cannot distinguish these species inhabiting on the same plate medium from each other by observation with the naked eye.

Some methods for detection and identification of Streptococcus mutans and Streptococcus sobrinus individually were reported. In these methods, these two kinds of bacteria are separated out of the oral streptococci including these two kinds of bacteria by means of a selective medium capable of substantially selecting these two kinds of bacteria alone, followed by biochemical or serological tests to distinguish these two species from each other (Gold, O. G. et al., 1973, Archs. Oral Biol. 18:1357–1364; Hirasawa, M. et al., 1980, Infect. Immun. 27:697–699; Kimmel, L. and N. Tinanoff, 1991, Oral Microbiol. Immunol. 6:275–279; Linke, H. A. B., 1977, J. Clin. Microbiol. 5:604–609; Schaeken, M. J. M. et al., 1986, J. Dent. Res. 65:906–908; Takada, K. et al., 1984, Infect. Immun. 45:464–469; and Wada, W. G. et al., 1986, J. Med. Microbiol. 22:319–323), or by a DNA prove method to distinguish them (Igarashi, T. et al., 1996, Oral Microbiol. Immunol. 5:294–298; and Shiroza, T. et al., 1998, Oral Microbiol. Immunol. 13:11–16). Further, some selective media for Streptococcus mutans have been reported (Gold, O. G. et al., 1973, Archs. Oral Biol. 18:1357–1364; Kimmel, L. and N. Tinanoff, 1991, Oral Microbiol. Immunol. 6:275–279; Linke, H. A. B., 1977, J. Clin. Microbiol. 5:604–609; Schaeken, M. J. M. et al., 1986, J. Dent. Res. 65:906–908; and Wada, W. G. et al., 1986, J. Med. Microbiol. 22:319–323). However, up to the present date, no selective culture technique capable of isolating Streptococcus sobrinus alone has been reported. Furthermore, the difference in the susceptibility to antibiotics between the different serotypes of mutans streptococci has been reported (Little, W. A. et al., 1979, Antimicrob. Agents Chemother. 15:440–443). However, no antibiotic which has the possibility to select Streptococcus sobrinus alone has not yet found.

DISCLOSURE OF THE INVENTION

The object of the present invention is to develop a method of selectively isolating Streptococcus sobrinus alone out of the population of oral streptococci including mutans streptococci, and a selective medium for selecting Streptococcus sobrinus.

In order to attain the above-mentioned object, the present inventors focused the susceptibilities of mutans streptococci to cell wall inhibitors, and tested the susceptibilities thereof to various types of antibiotics. As the results, the difference between Streptococcus mutans and Streptococcus sobrinus in their susceptibility to monobactam antibiotics was found, and the present invention was accomplished by the finding.

Namely, the present invention is to provide a method of substantially isolating Streptococcus sobrinus alone by the addition of a monobactam antibiotic to a medium on which oral streptococci including mutans streptococci can grow, and a selective medium for Streptococcus sobrinus prepared by the addition of a monobactam antibiotic to a medium on which oral streptococci including mutans streptococci can grow.

In the present invention, it is preferred that the medium on which only the oral streptococci can grow is Mitis Salivarius Agar (MS medium; manufactured by Difco).

In the present invention, as the monobactam antibiotic, aztreonam (AZT) or carumonam (CRMN) is preferred, and aztreonam (AZT) is particularly preferred. Here, the monobactam antibiotic is preferably added in a concentration of from 0.05 to 0.5 g/L.

The selective medium for Streptococcus sobrinus of the present invention preferably contains fosfomycin and/or bacitracin.

Further, it is particularly preferred that the selective medium for Streptococcus sobrinus of the present invention has the following composition:

Mitis Salivarius Medium 90 g/L Aztreonam 0.05–0.5 g/L Fosfomycin 0.05–0.5 g/L Bacitracin 5–50 units/L Sodium Chloride 5–40 g/L

BEST MODE FOR CARRYING OUT THE INVENTION

The method and selective medium of the present invention are characterized in that a monobactam antibiotic is added to a medium on which oral streptococci including mutans streptococci can grow.

In the background of the present invention, there is the fact that distinction of Streptococcus mutans and Streptococcus sobrinus on the medium on which the two kinds of bacteria inhabit together requires a lot of skill, since the two kinds of bacteria which attract the researchers attention as the major dental caries-causing bacteria among oral streptococci classified into the mutans streptococci, are extremely resemble to each other in their colony morphology and so on as is mentioned above. Here, mutans streptococci other than Streptococcus mutans and Streptococcus sobrinus have respective identifiable features in their colony morphologies and so on without requiring the particular skin, therefore, even if these bacteria inhabit together with Streptococcus mutans or Streptococcus sobrinus on the same medium, these two kinds of bacteria can easily be differentiated from the other bacteria.

In view of the above-mentioned background, in the present invention, the present inventors narrowed the focus to clearly differentiate Streptococcus mutans and Streptococcus sobrinus, searched for antibiotics to which significant difference between the two kinds of bacteria in their susceptibilities was observed, and prepared a selective medium for Streptococcus sobrinus, on which Streptococcus sobrinus alone could substantially grow.

Now, test conditions such as materials and methods used in the tests carried out in the research to accomplish the present invention will be described.

<Materials and Methods>

Bacterial Strains

All of the bacterial strains used in the research are shown in Table 1 described below.

TABLE 1 Strain Serotype <Mutans streptococci> Streptococcus cricetus OMZ-61 a E49 a Streptococcus rattus BHT b FA1 b Streptococcus mutans PS14 c Ingbritt c JC2 c NCTC10449 c LM7 e NUM-Sme1 e OMZ175 f NUM-Smf1 f Streptococcus sobrinus OMZ176 d B13 d NUM-Ssd1 d NUM-Ssd2 d NIDR6715 g OMZ65 g NUM-Ssg1 g Streptococcus downeii NCTC11391 h <Oral Streptococci> Streptococcus salivarius ATCC9222 HHT Streptococcus mitis NCTC3168 ATCC903 Streptococcus sanguis ATCC10556 Streptococcus oralis ATCC10557 Streptococcus gordonii ATCC10558 Challis Streptococcus anginosus ATCC9895 NCTC10709

Out of the above-described bacterial strains, the strains indicated by NUM-Sme, NUM-Smf, NUM-Ssd and NUM-Ssg were strains isolated from clinical samples, and identified by the combination of a colony morphological method and a serological method (Hamada, S. and H. D. Slade, 1980, Microbiological Review 44:331–384; Hirasawa, M. et al., 1980, Infect. Immun. 27:697–699; and Takada, K. et al., 1984, Infect. Immun. 45:464–469).

Out of the other bacterial strains, the strains to which “ATCC” is attached were purchased from American type Culture Correction, the strains to which “NCTC” is attached were purchased from National Collection Type Culture, and the other strains are those which have been preserved before in the laboratory to which the present inventors belong.

All the bacterial strains were grown and maintained anaerobically at 37° C. on Brain Heart Infusion (BHI broth, manufactured by Difco Laboratories, Detroit, Mich.) agar plates supplemented with 5% (v/v) horse blood.

Susceptibility Test to Antibiotics

The susceptibility test was carried out in accordance with microbroth dilution method.

In order to develop a selective medium for Streptococcus sobrinus, the preliminary study for selection of antibiotics was carried out by a disk susceptibility test (Sensi-Disc; manufactured by Becton Dickinson Co., USA) (no data given herein), and on the basis of the result of the preliminary study, the antibiotics used in the main test were determined. The antibiotics used in the main test were described below.

Cephaloridine (manufactured by Sigma Chemical Co., MO. USA)

Cefazoline (manufactured by Wako Pure Chemical Industries Ltd., Tokyo, Japan)

Cloxacillin (manufactured by Sigma)

Imipenem (manufactured by Banyu Pharmaceutical Co., Ltd., Tokyo, Japan)

Aztreonam (AZT) (manufactured by ICN Biomedicals Inc., OH, USA)

Carumonam (CAM) (manufactured by Takeda Chemical Industries Ltd., Osaka, Japan)

Cephalexin (manufactured by Sigma)

Benzylpenicillin (Wako Pure Chemical Industries Ltd.)

Cefoxitin (manufactured by Sigma)

The above-described antibiotics were dissolved in an appropriate solution, respectively, and the respective antibiotic solutions were divided into serial two-fold dilutions in a 96-well microtiter plates containing BHI broth. The bacteria were precultured in BHI broth, respectively, and inoculated into the wells containing the antibiotic solution to make a final concentration of the bacteria to be 1×10⁴ cells/well, respectively. The plates were anaerobically incubated at a temperature of 37° C. for 48 hours. After mixing the broth in each well by the use of a pipette, the absorbance at 550 nm in each well was measured with a microplate reader (manufactured by Corona Electric Co., Ibaraki, Japan), to determine bacterial growth. The antibiotic concentration at which no bacterial growth was observed was defined as the minimal inhibitory concentration (MIC).

Aliquots of 10 μl of the broth in the well, which no bacterial growth was observed were inoculated onto Mitis Salivarius Agar (MS medium; manufactured by Difco) plates, respectively, and anaerobically incubated at a temperature of 37° C. for 48 hours. The recovery of the inoculated bacteria was calculated to estimate which the mechanism of growth inhibition was bactericidal or bacteriostatic.

Results

Results of the susceptibility tests of Streptococcus mutans and Streptococcus sobrinus to β-lactam antibiotics are shown in Table 2 below.

TABLE 2 Susceptibilities of Streptococcus mutans and Streptococcus sobrinus to β-lactam antibiotics MIC(μg/mL) Streptococcus mutans Streptococcus sobrinus Antibiotic JC2(c)^(a) 10449(c) 0MZ176(d) B-13(d) Cephaloridine 0.025 0.025 0.025 0.025 Cefazoline 0.25 0.25 0.25 0.50 Cloxacillin 0.63 0.63 1.25 1.25 Imipenem 0.125 0.125 0.031 0.016 Aztreonam 125 125 2000 2000 Carumonam 125 125 2000 2000 Cephalexin 1.25 1.25 1.25 1.25 Benzylpenicillin 0.1 0.1 0.05 0.05 Cefoxitin 3.13 3.13 12.5 3.13 ^(a)Serotype given in parentheses.

From the results shown in Table 2, as to cepharoridine, cefazoline and cephalexin, no significant difference between Streptococcus mutans and Streptococcus sobrinus in their susceptibilities were observed.

As to cloxacillin, the susceptibilities of Streptococcus mutans were twice as high.

As to imipenem, the susceptibilities of Streptococcus sobrinus were approximately from 4 to 8 times as high as those of Streptococcus mutans.

As to benzylpenicillin, contrary to cloxacillin, the susceptibilities of Streptococcus sobrinus were twice as high.

As to cefoxitin, although only OMZ176 strain out of Streptococcus sobrinus indicated the susceptibility thereto approximately 4 times as high as those of the other bacterial strains, no significant difference between Streptococcus mutans and Streptococcus sobrinus in their susceptibilities was observed.

As to aztreonam and carumonam classified into monobactam antibiotics, the MICs of the both antibiotics to Streptococcus mutans were 125 μg/mL, respectively, while the MICs of these antibiotics to Streptococcus sobrinus were 2000 μg/mL, respectively, it becoming clear that the susceptibilities of Streptococcus mutans to these antibiotics were 16-fold as high as those of Streptococcus sobrinus. These observations indicate that the monobactam antibiotics are useful agents for separation of Streptococcus sobrinus from Streptococcus mutans.

It has been reported that imipenem, aztreonam and carumonam targeted the penicillin-binding protein-3 (PBP-3) with respect to E. coli (Georgopapadakou, N. H. et al., 1982, Antimicrob. Agents Chemother. 21:950–956; and Imada, A. et al., 1985, Antimicrob. Agents Chemother. 27:821–827), while with respect to Streptococcus mutans and Streptococcus sobrinus, imipenem and the monobactam antibiotics did not show the same mode of action.

Here, both aztreonam and carumonam had the mode of action of bactericidal to Streptococcus mutans and Streptococcus sobrinus.

Although in the above-described susceptibility tests to the antibiotics, only the two kinds of bacterial strains classified into the serotype c out of Streptococcus mutans, and only the two kinds of bacterial strains classified into the serotype d out of Streptococcus sobrinus were used, the susceptibilities of bacterial strains classified into the various serotypes to the monobactam antibiotics were studied. The results are shown in Table 3 below.

TABLE 3 Susceptibilities of bacterial strains classified into various serotypes of Streptococcus mutans and Streptococcus sobrinus to monobactam antibiotics MIC(μg/mL) Strain Serotype Aztreonam Carumonam Streptococcus mutans PS14 c 125 125 Ingbritt c 125 125 LM7 e 125 125 NUM-Sme1 e 125 125 OMZ175 f 250 250 NUM-Smf1 f 250 250 Streptococcus sobrinus NUM-Ssd1 d 2000  2000  NUM-Ssd2 d 2000  2000  NIDR6715 g 2000  2000  OMZ65 g 2000  1000 

From the results shown in Table 3, the bacterial strains classified into the serotypes c and e of Streptococcus mutans had MICs of 125 μg/mL, respectively, in the same as the results shown in Table 2. The bacterial strains classified into the serotype f of Streptococcus mutans had MICs of 250 μg/mL, respectively, and had the susceptibilities to aztreonam and carumonam lower than the bacterial strains classified into the serotypes c and e.

The bacterial strains classified into the serotypes d and g of Streptococcus sobrinus had MICs in the same as the results shown in Table 2, respectively, except that OMZ65 strain classified into the serotype g had an MIC to carumonam of 1000 μg/mL, it having the susceptibility twice as high as the other bacterial strains.

Also, both aztreonam and carumonam had the mode of action of bactericidal to the bacterial strains of Streptococcus mutans and Streptococcus sobrinus as shown in Table 3.

From the results shown in Tables 2 and 3, it became clear that the susceptibilities of Streptococcus sobrinus to the monobactam antibiotics were significantly lower than those of Streptococcus mutans. Therefore, it is understood that Streptococcus sobrinus can grow on a medium containing the monobactam antibiotics in such a concentration that Streptococcus mutans can not grow, and such a medium allow Streptococcus sobrinus alone to grow selectively. Namely, the present inventors have found that by the use of the monobactam antibiotics, Streptococcus sobrinus alone can selectively be isolated.

EXAMPLES

Now, the present invention will be specifically described with reference to examples. On the basis of the above-mentioned findings, the present inventors produced a trial model of a selective medium for Streptococcus sobrinus and evaluated it.

Preparation of Selective Medium

The selective medium for Streptococcus sobrinus having the composition shown in Table 4 below (hereinafter this medium will be referred to as MS-SOB medium.) was prepared.

TABLE 4 Composition of Selective Medium for Streptococcus sobrinus (MS-SOB medium) Component Mitis Salivarius Medium 90 g/L Aztreonam 0.2 g/L Fosfomycin 0.02 g/L Bacitracin 20 units/L Sodium Chloride 20 g/L

Mitis Salivarius medium (MS medium) in Table 4, manufactured by Difco, which is generally used for isolation of not only mutans streptococci but also the other streptococci, has the following composition.

Bacto Tryptose 10 g/L Bacto Proteose Peptone No. 3 5 g/L Bacto Proteose Peptone 5 g/L Bacto Dextrose 1 g/L Bacto Saccharose 50 g/L Dipottasium Phosphate 4 g/L Trypan Blue 0.075 g/L Bacto Crystal Violet 0.0008 g/L Bacto Agar 15 g/L

However, the base medium used for the selective medium for Streptococcus sobrinus of the present invention is not limited to the MS medium, and any media may be used on which only oral streptococci including mutans streptococci can grow.

Although aztreonam is used in the MS-SOB medium as the monobactam antibiotic, the monobactam antibiotic used in the present invention is not limited to aztreonam, and the antibiotics classified into the monobactam antibiotics, for instance, carumonam may be used. Further, the monobactam antibiotics available commercially in Japan at present are only two kinds of aztreonam and carumonam but the monobactam antibiotics used in the present invention are not limited to these two antibiotics. The concentration of the monobactam antibiotic are determined from the results shown in Table 2 and 3 within such a range that Streptococcus sobrinus can grow and the other oral streptococci including mutans streptococci can significantly be controlled. The concentration of the monobactam antibiotic is usually within a range of from 0.05 to 0.5 g/L, preferably from 0.1 to 0.3 g/L and particularly preferably 0.2 g/L.

Fosfomycin and bacitracin used here were purchased from Sigma. The mechanisms of action of fosfomycin and bacitracin are inhibitions of pyruvate N-acetylglucosamine-3-o-enol transferase and pyrophosphatase by binding to membrane phospholipids; respectively. Namely, the mechanisms of action thereof are to block from synthesizing cell wall peptide glycan. By the combined use of the monobactam antibiotics with fosfomycin and bacitracin which have the modes of action different from the monobactam antibiotics, it was expected that synergistic effect could be obtained. However, in the present invention, agents such as antibiotics to be added to the medium other than the monobactam antibiotic are not limited to fosfomycin and bacitracin. Fosfomycin is preferably used in a concentration of from 0.05 to 0.5 g/L, and bacitracin is preferably used in a concentration of from 5 to 50 units/L.

Sodium chloride is added to the medium for the purpose of adjusting osmotic pressure and the like, and the concentration thereof in the medium may optionally be determined on the basis of the relationship of the type, concentrations and so on of the other components, and is preferably within a range of from 5 to 40 g/L.

To the selective medium for Streptococcus sohrinus of the present invention, additional components capable of increasing the selectivity of Streptococcus sobrinus other than the components described in Table 4 above may be added.

The MS medium to which sodium chloride was added was sterilized and the medium was cooled to a temperature of 50° C. Then, the above-mentioned three kinds of antibiotics were added to the medium to prepare the MS-SOB medium.

Performance Evaluation Test of Selectivity to Streptococcus sobrinus

By the use of the MS-SOB medium and the MS medium selected as a medium on which oral streptococci including mutans streptococci can grow, the recoveries of Streptococcus sobrinus were obtained, to evaluate the performance of selectivity of the MS-SOB medium to Streptococcus sobrinus.

The bacterial strains indicated in Table 5 below were incubated anaerobically at 37° C. over night in advance. Ten-fold dilution of the culture solution was prepared by the addition of 0.9 mL of Tris-HCl buffer (0.05 M, pH 7.2) thereto, aliquots of 0.1 mL of the dilution were plated on the test media (MS-SOB and MS), respectively. The plates were incubated anaerobically at a temperature of 37° C. for 3 days, and then, colony forming units (CFU/mL) were counted. The results are shown in Table 5.

TABLE 5 Recoveries of Oral Streptococci and Mutans Streptococci on MS Medium and MS-SOB Medium Recovery (CFU %) Strain Serotype MS medium MS-SOB medium Streptococcus cricetus OMZ-61 a 1.3 × 10⁷ 1.8 × 10⁵ (13.5) E49 a 1.0 × 10⁷ 0 (0) Streptococcus rattus BHT b 1.1 × 10⁸ 0 (0) FA1 b 2.4 × 10⁸ 0 (0) Streptococcus mutans JC2 c 2.9 × 10⁸ 0 (0) NCTC10449 c 1.9 × 10⁸ 0 (0) LM7 e 2.4 × 10⁸ 0 (0) NUM-Sme1 e 3.0 × 10⁸ 0 (0) OMZ175 f 1.4 × 10⁸ 0 (0) NUM-Smf1 f 6.3 × 10⁷ 0 (0) Streptococcus sobrinus OMZ176 d 1.0 × 10⁷ 7.0 × 10⁶ (70.2) B13 d 5.8 × 10⁷ 4.7 × 10⁷ (81.3) NUM-Ssd1 d 1.4 × 10⁷ 1.2 × 10⁷ (84.8) NIDR6715 g 1.5 × 10⁷ 1.1 × 10⁷ (70.7) OMZ65 g 2.5 × 10⁷ 1.5 × 10⁷ (60.8) NUM-Ssg1 g 1.8 × 10⁸ 1.4 × 10⁸ (76.6) Streptococcus downeii NCTC11391 h 6.5 × 10⁸ 3.9 × 10⁸ (60.2) Streptococcus salivarius ATCC9222 2.0 × 10⁸ 0 (0) HHT 2.3 × 10⁸ 0 (0) Streptococcus mitis NCTC3168 9.6 × 10⁷ 0 (0) ATCC903 1.2 × 10⁸ 0 (0) Streptococcus sanguis ATCC10556 4.5 × 10⁸ 0 (0) Streptococcus oralis ATCC10557 2.4 × 10⁸ 0 (0) Streptococcus gordonii ATCC10558 9.4 × 10⁷ 0 (0) Challis 1.1 × 10⁸ 0 (0) Streptococcus anginosus ATCC9895 8.0 × 10⁷ 2.0 × 10³ (0.0025) NCTC10709 3.4 × 10⁸ 1.0 × 10³ (0.0003)

As is apparent from Table 5 above, all the bacterial strains used for the test grew well on the MS medium at a density of from 10⁷ to 10⁸ CFU/mL. On the other hand, the bacterial strains able to grow on the MS-SOB medium were limited only to Streptococcus cricetus OMZ-61 strain (recovery: 13.5%) out of the serotype a, all of the bacterial strains classified into the serotypes d and g of Streptococcus sobrinus (recovery: 74.1% on the average), Streptococcus downeii NCTC11391 strain of the serotype h (recovery: 60.2%), and Streptococcus anginosus ATCC9895 strain (recovery: 0.0025%) and NCTC10709 strain (recovery: 0.0003%).

Out of mutans streptococci able to grow on the MS-SOB medium, Streptococcus cricetus OMZ-61 strain of the serotype a is a rare bacterial strain detected in the oral cavity. Although Streptococcus downeii NCTC11391 strain of the serotype h was found at a high recovery of 60.2%, this bacterial strain has not been detected from human oral cavity up until now. Further, these bacterial strains can be distinguished from Streptococcus sobrinus in their colony morphological features. Therefore, it is vanishingly impossible to confuse these bacterial strains and Streptococcus sobrinus.

Streptococcus anginosus ATCC9895 strain and NCTC10709 strain belonging to the oral streptococci are extremely low in their recoveries, and they have features significantly different from Streptococcus sobrinus in their colony morphology. Therefore, even when these bacterial strains inhabit together with Streptococcus sobrinus, a person who is not conversant with their colony morphologies can certainly distinguished between Streptococcus sobrinus and these bacterial strains.

Out of the tested mutans streptococci, Streptococcus mutans which are hardly distinguished from Streptococcus sobrinus based on their colony morphologies can not grow on the MS-SOB medium at all, therefore, there is no possibility to confuse Streptococcus mutans and Streptococcus sobrinus.

From the above-mentioned facts, it is understood that by the use of the MS-SOB medium, Streptococcus sobrinus can be isolated selectively.

Confirmation of Selectivity of MS-SOB Medium using Clinical Samples

The selectivity of the MS-SOB medium was confirmed with respect to clinical samples obtained from the dental caries area in human oral cavity.

Sampling area of dental caries was wiped with a cotton roll, and saliva was removed from the sampling area with air blast for a short period of time. Then, sample was taken out from the area of dental caries with a sterilized dental probe. Each sample was taken into a sterilized micro-centrifuging tube containing 1 mL of 50 mM Tris-HCl buffer (pH 7.2). After ultrasonic dispersion, by the use of the same buffer, ten-fold serial dilutions of each sample were prepared, and appropriate dilutions were plated-triplicate on the MS plate medium and the MS-SOB plate medium. These plate media were incubated anaerobically at a temperature of 37° C. for 3 days, and then, recoveries (CFU%) were calculated. The samples containing bacteria in the total number of 1×10⁶ to 10⁸ on the average were used for clinical analysis.

Five colonies per a test subject, which seemed to be Streptococcus sobrinus in the light of their colony morphology, were subcultured to confirm the presence of Streptococcus sobrinus.

The pure culture strain from each separated strain was identified by 1) the colony morphology; 2) the fermentation pattern using their rapid ID 32 STREP (manufactured by bio Metrieux) or Streptogram (manufactured Wako Pure Chemical Industries Ltd.); and 3) agar gel immunodiffusion method using rabbit antiserum generated against the control strain prepared in our laboratory as mentioned above (Hirasawa, M. et al., 1980, Infect. Immun. 27:697–699; and Igarashi, T. et al., 1996, Oral Microbiol. Immunol. 5:294–298).

The clinical samples were divided into two groups of Group I having the ratio of Streptococcus sobrinus in all the streptococci of less than 1%, and Group II having the ratio of not less than 1%, on the basis of the number of existing Streptococcus sobrinus. All the samples contained the streptococci in the number of 10⁶ to 10⁷ on the average on the MS plate medium.

By the use of the above-mentioned clinical samples, the recoveries of Streptococcus sobrinus were calculated. The results are shown in Table 6 below.

TABLE 6 Recoveries of Streptococcus sobrinus on MS medium and MS-SOB medium from human dental caries samples Percentage of Streptococcus Number MS-SOB medium MS medium^(a) sobrinus of Streptococcus Streptococcus Group in all bacteria samples sobrinus Other bacteria sobrinus Other bacteria I Less than 1% 10 7.6 × 10^(3, b) 1.9 × 10 N.D.^(c) 6.2 × 10⁶ (4 × 10 to (0 to (5.0 × 10⁴ to 6.9 × 10⁴) 7.0 × 10) 3.0 × 10⁷) II Not less than 8 5.4 × 10^(6, a) 0 5.6 × 10⁶ 2.3 × 10⁷ 1% (1.8 × 10⁵ to (1.0 × 10⁵ to (6.5 × 10⁵ to 3.6 × 10⁷) 3.7 × 10⁷) 6.6 × 10⁷) ^(a)Mean number of CFU per mL on the triplicate plates containing 0.1 mL of a 10⁻⁴ or 10⁻⁵ dilution range ^(b)Mean number of CFU per mL on the triplicate plates containing 0.1 mL of an undiluted sample or a 10⁻¹ dilution ^(c)N.D.: Not detectable, plates too crowded. Number given in parenthesis indicates a range of detection.

As is apparent from the results of Table 6 above, in Group I, no Streptococcus sobrinus was detected on the crowded MS plate medium. On the other hand, on the MS-SOB medium, even less than 1% of Streptococcus sobrinus inhabited among the bacteria of the total number of from 10⁶ to 10⁷ could be detected. When the MS-SOB medium was used, the bacteria other than Streptococcus sobrinus were almost totally inhibited. The bacteria other than Streptococcus sobrinus, which grew on the MS-SOB medium, formed a small number of colonies or pinpoint colonies.

As for the samples of Group II, the pure strains of Streptococcus sobrinus were obtained on the MS-SOB medium, and the other bacteria (contaminants) did not grow on the MS-SOB medium. On the MS medium, Streptococcus sobrinus and the other bacteria inhabited together in almost the same quantity. It was difficult to find out Streptococcus sobrinus from the sample on the MS plate medium of Group II, which was contaminated by the other bacteria, as was expected.

From the results of Table 6 above, it is understood that by the use of the MS-SOB medium, Streptococcus sobrinus can selectively be isolated from clinical dental caries samples.

Study on Bacterial Growth on MS-SOB Medium with Respect to Clinical Dental Caries Sample

Sixty human clinical dental caries samples were incubated on the MS-SOB media, and the results of incubation are shown in Table 7 below. Here, all of these 60 samples contained bacteria in the number of 1×10⁷ on the average when measured on the MS plate media.

TABLE 7 Isolation of Streptococcus sobrinus on MS-SOB medium from 60 human clinical dental caries samples Growth^(a) Number of Samples Streptococcus sobrinus pure culture 14 Streptococcus sobrinus plus other bacteria 4 Other bacteria only >500 0  10 to 500 10  <10 9 No bacterial growth 23 ^(a)Mean number of CFU per mL on the triplicate plates containing 0.1 mL of an undiluted sample.

In 14 samples out of the 60 samples, Streptococcus sobrinus alone grew, and in 23 samples, no growth of any bacteria was observed.

In 4 samples, growth of Streptococcus sobrinus was mainly observed with the small number of colonies (number of colonies: 70 at the maximum) of the other bacteria (contaminants). These other bacteria could morphologically be distinguished from Streptococcus sobrinus.

Although growth of the other bacteria alone was observed in 19 samples, these bacteria formed their colonies in the number of less than 5×10² CFU/mL at the maximum. These other bacteria were, of course, morphologically distinguishable from Streptococcus sobrinus.

INDUSTRIAL APPLICABILITY

According to the present invention, a method of substantially isolating Streptococcus sobrinus alone selectively and a selective medium for Streptococcus sobrinus.

According to the present invention, there is no necessity to morphologically distinguish between Streptococcus mutans and Streptococcus sobrinus, which are closely resemble to each other in their morphologies so that the distinction of these species requires a lot of skill. Therefore, no confusion of these bacterial species could be brought about. Further, no confusion of Streptococcus sobrinus with oral streptococci including the mutans streptococci other than Streptococcus sobrinus could be brought about, too.

By the use of the selective medium of the present invention, Streptococcus sobrinus alone can selectively be isolated among oral streptococci including mutans streptococci and can be incubated, thereby it being possible to clarify the exact aspect of the presence of Streptococcus sobrinus in clinical dental caries samples.

If the aspect of the presence of Streptococcus sobrinus can be exactly gotten hold of, it becomes possible to clarify the significance and role of Streptococcus sobrinus in caries risk, which has been discussed in the art.

Furthermore, in the future, the distribution of Streptococcus sobrinus in oral cavity will be investigated and be able to be used for diagnosis of the caries risk of a patient. 

1. A method of substantially isolating Streptococcus sobrinus alone comprising adding a monobactam antibiotic to a medium on which only oral streptococci including mutans streptococci can grow; culturing a sample of the medium; and isolating Streptococcus sobrinus from the medium.
 2. The method according to claim 1, wherein the medium is Mitis Salivarius Agar.
 3. The method according to claim 1, wherein the monobactam antibiotic is aztreonam (AZT) or carumonam (CRMN).
 4. The method according to claim 3, wherein the monobactam antibiotic is aztreonam (AZT).
 5. The method according to claim 1, wherein the monobactam antibiotic is added to the medium in a concentration of from 0.05 to 0.5 g/L.
 6. The method according to claim 1, wherein the medium comprises fosfomycin, bacitracin and sodium chloride.
 7. The method according to claim 1, which comprises: Mitis Salivarius Medium 90 g/L Aztreonam 0.05 0.5 g/L Fosfomycin 0.05–0.5 g/L Bacitracin 5–50 units/L Sodium Chloride 5–40 g/L.


8. A selective medium for Streptococcus sobrinus prepared by adding a monobactam antibiotic to a medium on which only oral streptococci including mutans streptococci can grow.
 9. The selective medium for Streptococcus sobrinus according to claim 8, wherein the medium on which only oral streptococci can grow is Mitis Salivarius Agar.
 10. The selective medium for Streptococcus sobrinus according to claim 9, wherein the monobactam antibiotic is aztreonam (AZT) or carumonam (CRMN).
 11. The selective medium for Streptococcus sobrinus according to claim 10, wherein the monobactam antibiotic is aztreonam (AZT).
 12. The selective medium for Streptococcus sobrinus according to claim 8, wherein the monobactam antibiotic is aztreonam (AZT) or carumonam (CRMN).
 13. The selective medium for Streptococcus sobrinus according to claim 12, wherein the monobactam antibiotic is aztreonam (AZT).
 14. The selective medium for Streptococcus sobrinus according to claim 8, wherein the monobactam antibiotic is present in the medium in a concentration of from 0.05 to 0.5 g/L.
 15. The selective medium for Streptococcus sobrinus according to claim 8, wherein the selective medium further comprises fosfomycin and/or bacitracin.
 16. The selective medium for Streptococcus sobrinus according to claim 8, which comprises: Mitis Salivarius Medium 90 g/L Aztreonam 0.05–0.5 g/L Fosfomycin 0.05–0.5 g/L Bacitracin 5–50 units/L Sodium Chloride 5–40 g/L. 